(i) Field of the Invention
This invention relates to an optical pickup which records/reproduces data on/from a disc loaded in an optical disc device or the like, and an objective lens drive device including the optical pickup.
(ii) Description of the Related Art
A structure of optical disc devices will be described below.
An optical disc device includes a spindle motor for rotating an optical disc, an optical pickup for recording/reproducing information on/from the optical disc, and a sledge mechanism for moving the optical pickup in the radial direction of the optical disc. The optical disc device further includes control circuits individually controlling the sledge mechanism and a controller comprehensively controlling a reproduction signal from the optical disc or a record signal to the optical disc. One of the mechanisms which directly records/reproduces information on/from the optical disc is the optical pickup.
The operation of the optical disc device based on the optical pickup is described.
To read/write on the optical disc, a laser is emitted from the optical pickup onto the optical disc loaded in the optical disc device, and then is subjected to focus control of bringing a spot of the laser into focus on the data surface of the optical disc. Then, the disc is rotated for data track alignment to be similarly subjected to track control.
In this manner, the information on the optical disc is electrically converted to optical information through the optical pickup, which is then transmitted as data to the controller of the optical disc device. The optical disc device receives signals from the optical pickup in this manner and transmits/receives data to/from a personal computer or the like.
In recent years, the recording density of optical discs has been progressively increased. This has been realized mainly by shortening the laser wavelength. On the other hand, since the optical disc devices are used in personal computers and the like and development and popularity of the notebook type of personal computer has been accelerated, a reduction in thickness of the optical disc device is required.
In turn, a reduction in thickness of an optical pickup used in the optical disc device is absolutely necessary. Since an optical pickup supporting high density discs has wavelength incompatibility with a conventional scheme, two objective lenses are mounted on an object lens drive device. A reduction in thickness of the optical pickup must be implemented with this structure.
For driving an object lens, a magnetic propulsion force is generated at a coil mounted on a movable section of the objective lens drive device by magnetic action of the coil and a magnet mounted on a stationary section located opposite to the coil. The magnetic propulsion force serves as a force that drives the objective lens to follow the wobble of a data track during the rotation of the optical disc. A wobbling direction of the data-track wobble is mainly the out-of-plane direction of the optical disc, that is, a focusing direction, and the radial direction of the optical disc, that is, a tracking direction. Because of this, the objective lens drive device is equipped with a focusing coil and a tracking coil in the movable section.
JP-A No. 2005-129191 (page 14, FIG. 1) discloses an example of such objective lens drive devices. The objective lens drive device disclosed in JP-A No. 2005-129191 includes a drive coil comprising a focusing coil wound around an objective lens holder, a tracking coil attached to the surface of the focusing coil, and a magnet disposed such that a magnetic field is induced on the surface opposite to the tracking coil.
JP-A No. 2006-120205 (FIG. 1) provides a low-profile optical pickup structured such that focusing and tracking coils are arranged on the right and left sides of an objective lens.
JP-A No. 2005-129191 relates to a magnetic circuit configuration in an objective lens drive device of an optical pickup, and particularly describes a coil configuration. For an optical pickup for a half-height type (device height is about 45 mm), the size of a magnet can be increased in accordance with a size of the optical pickup. This makes it possible to generate a necessary sufficient magnetic propulsion force at each of the focusing and tracking coils.
However, achievement of both low-profile styling and placement of two objective lenses requires a reduction in size of the magnet. In step with this, the coil must be reduced in size. As a result, a sufficient magnetic propulsion force is generated at the coil with difficulties.
JP-A No. 2006-120205 describes an ultra-thin pickup which is equipped with one objective lens. The objective lens is placed at a center of the objective lens holder. Therefore, the focusing coil and the tracking coils can be arranged on the right and left sides of the objective lens.
However, in the case of mounting two objective lenses, if the two objective lenses are placed in an objective lens holder with the same dimensions as the above-described one, symmetric arrangement of the focusing coil and the tracking coils is made impossible. Also, an increased number of objective lenses leads to an increases in weight, which in turn causes a reduction in acceleration sensitivity of the objective lens drive device. Further, in recent years, optical discs have been used as a distribution medium of a published book. The stacked books or the like apply pressure to the optical discs, so that the optical discs may be deformed, in particular, in the out-of-plane direction. If information is reproduced or the like from such a deformed optical disc, displacement occurs, particularly, in the focusing direction. Because of this, the acceleration sensitivity in the focusing direction is required to be increased. Considering this circumstance, in the configurations of JP-A No. 2005-129191 and JP-A No. 2006-120205, insufficient acceleration sensitivity is pronounced.
Accordingly, it is necessary to study a structure of an objective lens drive device of an optical pickup which is adapted to an ultra-thin type of optical pickup equipped with two objective lenses and makes the faster performance of optical discs feasible.